Can anyone explain more for me Newton's 3rd Law of Motion?

In summary, according to Newton's Third Law, for every force applied to object A by another object B, there is an equal and opposite force(reaction) applied to object B by object A. This law is specifically for two objects that are in contact with each other. If an astronaut is in outer space, he would experience the force of gravity from the earth, but also the force of the atmosphere (or space) acting against him.
  • #1
zeo2396
9
0
As I learn about Newton's Third Law:For every force applied to object A by another object B, there is an equal and opposite force(reaction) applied to object B by object A.

I think if we had two equal and opposite forces exerts on a thing so it will not move!

Also, if there is an astronaut in outer space, what type of 2 forces can exerts on him? How can I recognize what force(action) exerts an equal and opposite on the second force (reaction) according to Newton'3rd Law?

Please help me to resolve these confuse!
Thank you!
 
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  • #2
zeo2396 said:
As I learn about Newton's Third Law:For every force applied to object A by another object B, there is an equal and opposite force(reaction) applied to object B by object A.
yes, that's right
I think if we had two equal and opposite forces exerts on a thing so it will not move!
and this is wrong. Note from your first statement, the equal and opposite force acts on a different object, not the same object.
Also, if there is an astronaut in outer space, what type of 2 forces can exerts on him? How can I recognize what force(action) exerts an equal and opposite on the second force (reaction) according to Newton'3rd Law?

Please help me to resolve these confuse!
Thank you!
You are confusing Newton 3 with his other 2 laws. The first 2 laws state that a net unbalanced force acting on an object will accelerate that object in the direction of the net force, if there is one. The third law states what you said earlier, and applies to different objects. If a man is standing on a table, he doesn't move, because the weight force acting down on him by the earth, is balanced by the normal force of the table acting up on him. That's 2 forces (the gravity force of the Earth and the normal force of the table) acting on one object. Newton 3 says in this situation that since the table exerts a normal force on the man, the man exerts an equal and opposite normal force on the table; and since the Earth is exerting a force on the man, the man exerts an equal and opposite force on the earth. Each of these force pairs act on different objects.

Now suppose the table breaks and the man falls. Now there is just one force acting on the man, the Earth force of gravity, so the man must accelerate down, per Newton 2. Newton 3 says in this case, that the eatrth is exerting a force on the man, and the man is exerting an equal and opposite force on the earth. Of course, the man is still moving, due to Newton 2.
 
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  • #3
PhanthomJay said:
yes, that's rightand this is wrong. Note from your first statement, the equal and opposite force acts on a different object, not the same object.You are confusing Newton 3 with his other 2 laws. The first 2 laws state that a net unbalanced force acting on an object will accelerate that object in the direction of the net force, if there is one. The third law states what you said earlier, and applies to different objects. If a man is standing on a table, he doesn't move, because the weight force acting down on him by the earth, is balanced by the normal force of the table acting up on him. That's 2 forces (the gravity force of the Earth and the normal force of the table) acting on one object. Newton 3 says in this situation that since the table exerts a normal force on the man, the man exerts an equal and opposite normal force on the table; and since the Earth is exerting a force on the man, the man exerts an equal and opposite force on the earth. Each of these force pairs act on different objects.

Now suppose the table breaks and the man falls. Now there is just one force acting on the man, the Earth force of gravity, so the man must accelerate down, per Newton 2. Newton 3 says in this case, that the eatrth is exerting a force on the man, and the man is exerting an equal and opposite force on the earth. Of course, the man is still moving, due to Newton 2.

But if the astronaut is floating in the space, do you know what forces can exert on him and the other opposite force (reaction) (it's not gravity right?)

So, back to your example, if there is a man is standing on a table, he exerts a force on a table and a table also exerts an opposite equal force on him, so can you show me how to identify what force is action and what force is reaction (the man's force exerts or the table's force exerts).

I'm not sure I am right or wrong, but if I throw a boy to my friend, and he steps back a little bit, is it right that the ball's force exerts on my friend is action, and the force my friend exerts on a ball is reaction! I still have some confuse to identify action and reaction force!
Thanks.:biggrin:
 
  • #4
zeo2396 said:
But if the astronaut is floating in the space, do you know what forces can exert on him and the other opposite force (reaction) (it's not gravity right?)
Forget this action-reaction terminology. If an astronaut is floating in space, ther is just one force acting on him...and that would be his weight (the force of the Earth on him, the gravity force). The net force acting on him is the gravity force, since it is the only force acting on him. It causes him to fall, per Newton 2, but since he is moving tangentially around the Earth at tremendous speed, he is moving in a circle as he is falling, so he stays in orbit. Now also, since the Earth is exerting a force on him, he is exerting a force on the earth. Per Newton 3. A floating astronaut is a bad example to discuss Newton 3. A better example would be if one astronaut A pushed on another astronaut B ...If Astonaut A pushes on B, B moves and accelerates, per Newton 2, and since A is pushing on B, then B is pushing on A with an equal and opp force, per Newton 3, so A moves and accelerates also, the other way, per Newton 2.
So, back to your example, if there is a man is standing on a table, he exerts a force on a table and a table also exerts an opposite equal force on him, so can you show me how to identify what force is action and what force is reaction (the man's force exerts or the table's force exerts).
I guess that's why i don't like the action-reaction terminology. Ignore it.
I'm not sure I am right or wrong, but if I throw a boy to my friend, and he steps back a little bit, is it right that the ball's force exerts on my friend is action, and the force my friend exerts on a ball is reaction! I still have some confuse to identify action and reaction force!
Thanks.:biggrin:
Right...who or what is acting and who or what is reacting...and who cares?:wink:
 
  • #5


Sure, I would be happy to explain more about Newton's Third Law of Motion. This law states that for every action, there is an equal and opposite reaction. This means that when an object exerts a force on another object, the second object will exert an equal and opposite force back on the first object. This law applies to all types of forces, including gravitational, electrical, and mechanical forces.

In terms of your question about the astronaut in outer space, there are two types of forces that could exert on the astronaut: contact forces and non-contact forces. Contact forces occur when two objects physically touch each other, while non-contact forces occur when two objects do not touch but still exert a force on each other, such as gravity.

To identify the equal and opposite forces acting on the astronaut, we can look at the types of forces present. For example, if the astronaut is standing on a surface, there will be a contact force between the astronaut and the surface due to their physical contact. This force will have an equal and opposite reaction force from the surface pushing back on the astronaut.

If the astronaut is floating in space, there may be non-contact forces acting on them, such as gravitational forces from nearby planets or stars. In this case, the astronaut's body will exert a gravitational force on the object they are near, and that object will exert an equal and opposite gravitational force back on the astronaut.

I hope this helps to clear up any confusion you may have about Newton's Third Law of Motion. If you have any further questions, please don't hesitate to ask. Keep learning and exploring the fascinating world of science!
 

1. What is Newton's 3rd Law of Motion?

Newton's 3rd Law of Motion states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another object, the second object will exert an equal force in the opposite direction on the first object.

2. How does Newton's 3rd Law of Motion apply to everyday situations?

An example of Newton's 3rd Law in everyday life is when you push a door open. Your hand exerts a force on the door, and the door exerts an equal force in the opposite direction, pushing your hand back. Another example is when you jump off a diving board. The force of your feet pushing down on the board causes the board to push up on your feet with an equal force, propelling you into the air.

3. How is Newton's 3rd Law related to the other laws of motion?

Newton's 3rd Law is closely related to the other two laws of motion. The first law, also known as the law of inertia, states that an object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by an external force. The second law, also known as the law of acceleration, states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. These laws work together to explain the motion of objects in the universe.

4. Can you give an example of Newton's 3rd Law in the field of physics?

In the field of physics, Newton's 3rd Law is crucial in understanding how rockets are propelled into space. The rocket engines push hot gases out of the back of the rocket with a great amount of force, causing the rocket to move in the opposite direction with an equal force. This is known as action and reaction forces, and it allows the rocket to overcome the force of gravity and launch into space.

5. How did Newton discover the 3rd Law of Motion?

Isaac Newton discovered the 3rd Law of Motion while studying the motion of objects and the effects of forces on them. He observed that every action had an equal and opposite reaction, leading him to formulate this law. This discovery, along with his other laws of motion and theory of gravity, revolutionized the field of physics and had a significant impact on our understanding of the universe.

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